The present invention relates generally to erasable inks and more particularly to erasable inks suitable for use in roller-ball or ball-point pens and to roller-ball and ball-point pens containing erasable inks.
When using writing instruments to produce written images, characters or highlight marks on a marking substrate, it is not necessarily desirable to obtain a high degree of indelibility. For example, one may want to be able to make a mark on a marking substrate which is easily erasable therefrom by substantially non-abrasive mechanical means, such as by an ordinary pencil eraser, a dry cloth, etc. To be truly erasable, the ink must be capable of being substantially completely removed from the marking substrate to which it has been applied without causing significant damage to the marking substrate. For the reasons to be discussed below, this goal is often particularly difficult to attain when the marking substrate is porous, as in the case of conventional stationery paper.
Conventional stationery paper is essentially a mat of randomly oriented cellulose fibers. Thus, conventional stationery paper consists of solid structural members, namely cellulose fibers, having numerous minute voids therebetween. From the foregoing, it may readily be seen that the formulator or designer of an erasable ink must prevent the colorant portion of the ink from penetrating, to any substantial degree, into the voids in the surface being written upon because, if there is substantial penetration of the colorant portion of the ink into the voids, the subsequent removal of the colorant by mechanical means cannot be accomplished without causing damage to the writing surface. Likewise, it may readily be seen that, for a writing medium to be erasable, the colorant must be prevented from affixing itself to any substantial degree of permanence, either by chemical reaction or as a result of mere physical attraction, to the solid members of the paper substratum. The foregoing considerations tend to suggest that an erasable ink should comprise relatively large colorant particles which are incapable of penetrating the voids of a paper substrate. However, this approach to achieving erasibility has not been very successful, particularly where the ink has been dispensed from a roller-ball or ball-point pen, since inks containing relatively large colorants often have difficulty in being passed through the very small clearance between the ball and socket of a roller-ball or ball-point pen.
Another approach that has been used in an effort to achieve erasability in a roller-ball or ball-point pen has been to include a mechanically-strippable film-forming or resinous material in the ink that prevents the colorant from penetrating the voids in the paper and from becoming affixed to the solid members of the paper substratum. One class of such inks are organic or oil-based inks having a high viscosity e.g., at least several thousand cps, and are used in ball-point pens. An example of the foregoing type of ink is described in U.S. Pat. No. 4,407,985, inventor Muller, which issued Oct. 4, 1983 and which is incorporated herein by reference. The Muller ink composition is said to be characterized by an initial, yet transient, erasability from an absorbent paperlike writing surface by ordinary pencil erasers when applied by a ball-point writing instrument. The Muller ink composition comprises polar elastomers, such as butadiene/acrylonitrile copolymer rubbers and vinyl acetate/ethylene copolymer rubbers, and mixtures thereof, either alone or in admixture with cis-1,4-polyisoprene (natural rubber or synthetic rubber), pigment, and a solvent system. The polar elastomer is present in the range of about 14-25% by weight of the ink and the pigment is present in amount sufficient to impart color and intensity to a trace of the ink when applied to a substrate such as paper. The solvent system essentially comprises a volatile component having a boiling point less than about 180.degree. C. and an essentially non-volatile component which includes pigment vehicle having a boiling point greater than about 300.degree. C. and low viscosity plasticizer having a boiling point greater than 180.degree. C. but less than 300.degree. C., the solvent system being present in amount sufficient to impart a final ink viscosity in the range of about 1.5 million cps.
Another example of an oil-based erasable ink used in a ball-point pen is described in U.S. Pat. No. 4,687,791, inventors Miyajima et al., which issued Aug. 18, 1987, and which is incorporated herein by reference. The Miyajima ink comprises a rubber component, a volatile solvent, a pigment and a nonvolatile solvent, which additionally comprises finely divided powders of an inorganic compound inactive to other ink components contained therein, a semisolid substance, a soft solid substance having a melting point not higher than 130.degree. C., a combination of finely divided powders of an inorganic compound inactive to other ink compositions contained therein and a semisolid substance, or a combination of finely divided powders of an inorganic compound inactive to other ink compositions contained therein and a soft solid substance having a melting point not higher than 130.degree. C.
One problem that has been noted generally in connection with organic or oil-based inks, whether erasable or not, is that such inks, when employed in a ball-point pen, often do not produce a pleasant-feeling writing effect due to the comparatively large amount of force necessary to dispense the highly-viscous organic ink through the small clearance between the ball and socket of the pen. In addition, such inks, due to their inherent fluid characteristics, often have a tendency to produce globs and skips when dispensed from ball-point pens. Such inks also tend to bleed unacceptably through conventional stationery paper and often possess toxicity and a noxious odor. On the other hand, aqueous-based inks, whether erasable or not, typically have a comparatively lower viscosity than do oil-based inks and tend to avoid many of the problems associated with oil-based inks; however, also as a result of their comparatively low viscosity, aqueous-based inks cannot typically be used in conventional ball-point pens. For this reason, aqueous-based inks have traditionally been dispensed from roller-ball pens comprising the combination of an absorbent filler and one or more absorbent feed rods ("nibs"). (See, for example, U.S. Pat. No. 4,145,148, inventor Fukuoka, which issued Mar, 20, 1979, and which is incorporated herein by reference.) The filler is typically disposed in the reservoir of the pen, and the nibs typically interconnect the reservoir to the environs of the ball and socket of the pen. Alternatively, aqueous-based inks have also been dispensed from roller-ball pens lacking a filler but comprising one or more nibs. Recently, however, for example, in U.S. Pat. Nos. 4,671,691, 4,686,246, 5,013,361 and 4,786,198, all of which are incorporated herein by reference, there are disclosed aqueous-based, non-erasable inks suitable for use in filler-less, nibless, roller-ball pens. Such inks include a material which endows the inks with the property of being shear-thinning, i.e., the inks are comparatively highly viscous at rest (although less viscous than the inks traditionally used in ball-point pens), but when subjected to the high shear rates produced during writing, become substantially less viscous, e.g., less than 100 cps. Examples of shear-thinning materials disclosed in the aforementioned patents include gums, such as xantham gum.
In U.S. Pat. No. 5,217,255, inventors Lin et al., which issued Jun. 8, 1993, and which is incorporated herein by reference, there is disclosed an erasable system including an element having a porous marking surface and an erasable marking composition for applying markings to the porous marking surface. The Lin et al. marking composition comprises an aqueous dispersion of particles of film-forming polymeric materials and a colorant, the Lin et al. marking composition being said to provide an erasable, substantially water-insoluble coalesced residue on a porous marking surface. Especially preferred polymeric materials for the Lin et al. marking composition include copolymers of styrene, such as styrene-butadiene. PLIOLITE 3757, which Lin et al. describes as a styrene-butadiene latex available commercially from Goodyear Tire and Rubber Company and containing about 70% by weight styrene-butadiene, is identified by Lin et al. as an example of a suitable polymeric film-forming material. The Lin et al. marking composition may include ingredients for controlling or adjusting the rheological properties of the marking composition. Such ingredients are said to include thixotropic or gelling agents such as clays, silicas or water-insoluble, water-soluble or water-miscible polymeric binder materials or polymeric shear thinning providing materials, such as xanthan gum, among others. Xanthan gum is said to be the preferred material for controlling rheological properties, and preferred amounts of xanthan gum are amounts between about 0.05 to about 2 percent by weight of the composition.
In U.S. Pat. No. 5,389,717, inventors Santini et al., which issued Feb. 14, 1995, and which is incorporated herein by reference, there is disclosed a marking composition that is said to be erasable for an extended period of time from a porous marking substrate using a common pencil eraser. The Santini et al. composition comprises a noncarboxylated styrene-butadiene copolymer resin, a colorant and an aqueous solvent. PLIOLITE.RTM. 2108, which is said to have a styrene-butadiene ratio of 29:71 and which is said to be commercially available from Goodyear Tire and Rubber Company, is said to be a particularly preferred resin for use in the Santini et al. composition. The Santini et al. composition may further include additives, such as coalescing agents, water-soluble or associative thickeners, release agents, surfactants which are preferably non-carboxylated, and the like. The Santini et al. composition is said to be suitable for use in porous-tipped writing instruments, such as felt tipped markers; however, the Santini et al. composition is said not to be limited to such markers and may be used in ballpoint and fountain pens, as paints for application with brushes, rollers, air brushes and the like.
In commonly-assigned PCT Application No. PCT/US93/10231, which was published on May 11, 1994 under International Publication Number WO 94/10251 and which is incorporated herein by reference, there is disclosed an erasable writing medium composition suitable for use in porous tip pens and roller-ball pens. According to one embodiment, the composition is adapted for use in fillerless, nibless, roller-ball pens and comprises a non-carboxylated styrene butadiene latex emulsion, a water-insoluble pigment and tragacanth gum. The composition may further include additives such as a fluorochemical surfactant and a 2:1 mixture of Sorbitol and glycerine. The aforementioned patent also discloses using a ceramic ball, instead of a metal ball, in a roller-ball or ball-point pen.
One problem that the present inventors have observed when aqueous-based erasable inks of the type described above are used in conventional roller-ball pens is that such inks tend to corrode those components of the roller-ball pen contacted therewith. Another problem is that such inks tend to be unduly unstable and are easily agglomerated by shear or pH stresses. Such stresses are common during the mixing of such inks, during the filling of roller-ball pens with such inks and during the dispensing of such inks using roller-ball pens. Still another problem is that such inks tend to exhibit less than optimal performance in terms of writability, erasability and "cap-off" (the period of time after which an uncapped pen becomes unusable due to the drying of ink on the tip of the pen).